LanguageClient: reset project pointer when project is closed

Change-Id: I9a2787404bd47bf338125df1066d88e9061b5768
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/languageclient/client.cpp

@@ -813,6 +813,7 @@ void Client::projectClosed(ProjectExplorer::Project *project)
             m_state = Shutdown; // otherwise the manager would try to restart this server
             emit finished();
         }
+        m_project = nullptr;
     }
 }